1. Field of the Invention.
This invention relates to compressor controls, and in particular to a valve for communicating a compressor cylinder with one of a clearance pocket and a suction chamber.
2. Description of the Prior Art.
Compressor controls are well-known in the field of compressors, and a variety of different designs have been developed in an attempt to achieve reliable and effective compressor control with minimal effect on overall compressor performance when fully loaded and with minimal complexity of structure.
One type of prior art compressor unloader utilizes mechanical fingers or valve lifters which physically engage and hold the sealing elements of a valve structure in their open positions, whereby no compression can occur. For example, see the Callan U.S. Pat. No. 1,275,843, the Haight U.S. Pat. No. 1,470,077 and the Redfield U.S. Pat. No. 1,505,604, all of which disclose the use of such mechanical fingers or valve lifters for holding plate valves open. Although virtually infinite clearance may be provided through the use of such a system, one of its disadvantages is a loss of compressor efficiency in fully loaded operation due to losses caused by the placement of the finger mechanisms within the valve flow passages. Another disadvantage is that because the fingers must physically contact the sealing elements in a valve, these parts are particularly susceptible to breakage and such unloader systems are therefore characterized by relatively high maintenance costs and down time.
In the case of gas compressors used for the transmission of natural gas, such breakdowns are relatively costly. The entire compressor must usually be shut down so that the damaged unloader mechanisms can be repaired or replaced, which generally requires removing a respective suction valve. The down time of such a compressor represents lost revenue from the natural gas which would otherwise be transmitted during such down time. Also, the natrual gas within the compressor when it is shut down is generally lost to the atmosphere.
To avoid the problems associated with using mechanical fingers or valve lifters for holding valve members in their open positions, compressor unloaders have been devised which utilize pistons sliding within sleeves and thereby opening the compressor cylinder to a clearance pocket or a suction chamber. This type of compressor unloader apparatus is exemplified in our U.S. Pat. No. 4,043,710 wherein a suction valve has a central passage therethrough with an unloader piston slidably disposed therein. A valve is provided for opening a clearance bottle to the compressor cylinder, and the unloader piston is movable between first and second positions whereby the compressor cylinder may be opened to a suction chamber.
Such hollow unloader pistons slidably mounted in sleeves are generally simpler and more reliable than the previously utilized mechanical finger or valve lifter type unloaders. Furthermore, if the unloader pistons are hollow, they offer less area to the compressed gas within the compressor cylinder, are less susceptible to buffeting and may be actuated by actuator mechanisms requiring relatively little force. However, in valves equipped with such unloaders, certain amounts of additional clearance space are formed by the valve central passage and a central passage through such a hollow unloader piston. The total capacity of the compressor when fully loaded is therefore reduced by the introduction of this additional clearance space.
A method of unloading a compressor with an unloader piston slidable in a sleeve communicating with a suction valve passage is exemplified in the Degroff et al U.S. Pat. No. 3,518,032. Because the unloader piston disclosed therein is substantially solid, relatively little additional clearance or head space is added to the compressor cylinder. However, the unloader piston of that device presents a substantial area to the compressor cylinder and is therefore much more susceptible to buffeting by the rapidly increasing and decreasing gas pressures. An actuator having considerable power is thus required to slide the unloader piston.
Prior art compressor controls have therefore tended to be complex either in structure or in manufacture, susceptible to breakdowns, relatively expensive, operated with extensive and powerful actuator means, or tended to reduce the compressor capacity.